There are a number of regulations relating to pressure pipe made from polymers and in particular polyethylene. There are a number of ASTM methods for testing the crack growth and the strength of pipe under stress (ASTM F1474 (corresponding to ISO 13479), D2837; and D1598), additionally there is the Pennsylvania Notched Impact test (PENT) (ASTM F 1473). Generally, the industry associations and government regulators specify a PENT value for pressurized pipe made from a resin. To determine a value for the PENT test a pipe is extruded from the resin and then subjected to a long term pressure test. The time to do this test may be in the order of thousands of hours. In the manufacture of pipe for these applications manufacturers rely on the manufacturing specifications and regular testing of resins to assure that resins made within the manufacturing specification will meet a required PENT value. There is a need for a simple relatively quick test procedure that can be carried out at a manufacturing site to determine (estimate within 10%) the PENT value of a resin.
U.S. Pat. No. 6,935,185 issued Aug. 30, 2005 in the name of Corleto, assigned to Fina Technology, Inc. discloses an accelerated method to determine the failure time of a polyethylene using a notched stress test and determining the minimum displacement rate for the polymer. This is the point at which there is slow crack growth due to crazing at the tip of the notch. The patent teaches away from the present disclosure as it does not teach or suggest a differential scanning calorimetery (DSC) method to predict PENT values.
The present invention provides a relatively simple procedure to predict a PENT value for a resin with a high degree of certainty. (e.g. 90% certainty).